This invention relates to a wavelength-division multiplexing optical transmission system. More particularly, the invention relates to a wavelength-division multiplexing optical communication system in which the capacity and transmission distance of an optical transmission system are increased by suppressing interchannel variations (inter-wavelength variations) in received light power, which are caused by wavelength-dependent gain of optical amplifiers and wavelength-dependent loss in the optical fiber of the transmission line.
Interchannel variations (inter-wavelength variations) in the power of received light in a WDM optical amplifying repeater transmission system are caused by the characteristics and wavelength dependence of an optical lossy medium (optical devices, optical amplifiers and the optical transmission line) through which the wavelength-division multiplexed signal is transmitted, and may be classified into the following components depending upon the cause and characteristics:
1) a slope (primary slope) component caused by the wavelength-dependent loss of the optical transmission line and optical devices and by the Raman effect of the optical transmission line;
2) a beat component (a comparatively gentle rise and fall in the shape of the spectrum) caused by the wavelength-dependent gain of the optical amplifiers; and
3) a ripple component (a deviation on the order of 0.1 to 1 nm) caused by a gain equalizer in the optical amplifiers, an optical device used in an OADM (Optical Add/Drop Multiplexer), etc., a deviation in the output level of the transmitter in each channel and an adjustment error following wavelength-division multiplexing.
Interchannel variation in optical power at. The receiving end that is the result of these factors produces variations in optical SNR and, as a result, degrades the transmission characteristic (bit error rate, or BER) and imposes a severe limitation upon the capacity and transmission distance of WDM optical transmission. More specifically, since the wavelength signal of lowest power among the multiplexed wavelength signals is the lower-limit value of receive power after transmission, the maximum transmission distance is limited by the wavelength signal of lowest power. Accordingly, reducing the variation between wavelengths after transmission is critical in terms of enlarging the maximum relay transmission distance.
To achieve this, measures and control set forth below have been adopted to minimize interchannel variations.
1) Slope, beat and ripple components are compensated for by optical pre-emphasis control.
2) The slope component is compensated for by slope compensation control by an optical amplifier (EDFA) constituted by a rare-earth doped fiber.
3) Slope and beat components are compensated for by slope compensation control and flattening control of Raman amplifiers.
Pre-Emphasis Control
Optical pre-emphasis control is a function for measuring or calculating the SNR of receive light at the receiving end and adjusting transmit power at the transmitting end so as to equalize the optical SNRs. FIG. 22 is a diagram useful in describing optical pre-emphasis control (see the specification of Japanese Patent Application Laid-Open No. 2001-203414). A WDM optical signal generated by an optical transmitter 11a in an optical transmitting station 11 is amplified by a plurality of optical repeaters 13a, 13b, . . . 13n, which are provided in optical transmission lines 12, so as to compensate for loss along the optical transmission lines 12 and loss in the optical repeaters 13a, 13b, . . . 13n, the amplified signal is transmitted to an optical receiving station 14 and the signal is received and processed by an optical receiver 14a. Loss in the optical repeaters 13a, 13b, . . . 13n is produced by optical component parts such as a dispersion compensating fiber used in the stations.
When the WDM optical signal is sent from the optical transmitter 11a to the optical transmission lines 12, pre-emphasis is applied by a pre-emphasis control circuit 11b within the optical transmitting station. That is, the pre-emphasis control circuit 11b calculates the difference between an average value of optical SNRs of all channels received from the optical receiving station 14 and the optical SNR of each individual channel and adjusts the optical level of each channel so as to compensate for this difference. The optical transmitter 11a wavelength-division multiplexes the adjusted optical signals of all channels and sends the multiplexed signals to the optical transmission lines 12. The optical SNR of the optical signal of each wavelength is measured by an optical-SNR measurement circuit 14b, e.g., a spectrum analyzer, provided in the optical receiving station 14, the information concerning the SNR is transmitted to the optical transmitting station 11 via a line 15 and then the above-described pre-emphasis control is repeated. As a result of the above operation, control is exercised in the optical receiving station 14 so as to uniformalize SNR.
Slope Compensation Control by Optical Amplifiers
Slope compensation control at an optical amplifier is a function for transmitting light provided with slope beforehand in such a manner that the wavelength characteristic of the input signal (spectrum) to the optical amplifier of the next stage will be flattened, this being performed based upon information concerning the multiplexing-number of wavelengths input to the optical amplifier and distance information relating to the transmission-line fiber length up to the destination of transmission (see the specification of Japanese Patent Application No. 2001-244528). FIG. 23 is a block diagram of an optical amplifier having a slope compensating function. When an optical amplifier 20 amplifies light entrant from an optical transmission line 21 and outputs the amplified light to an optical transmission line 22, the optical amplifier 20 performs amplification with a gain wavelength characteristic that compensates for the loss wavelength characteristic possessed by the optical transmission line 22. More specifically, in a case where light over a certain wavelength band exits from the optical amplifier 20, the optical amplifier 20 delivers the light so as to compensate beforehand for the difference between loss on the short-wavelength side and loss on the long-wavelength side of the light on the optical transmission line 22. The loss discrepancy is caused by the loss wavelength characteristic of the optical transmission line 22 during transmission of the light over this optical transmission line. The optical amplifier 20 applies compensation in advance upon making the gain wavelength characteristic a characteristic that is substantially the inverse of the loss wavelength characteristic of the optical transmission line 22.
The optical amplifier 20 includes first optical amplifying means 20a for amplifying light; optical attenuating means 20b for attenuating the light amplified by the first optical amplifying mans 20a; second optical amplifying means 20c for amplifying the light attenuated by the optical attenuating means 20b and outputting the amplified light to the optical transmission line 22; and control means 20d for adjusting the amount of attenuation in the optical attenuating means 20b in such a manner that the gain wavelength characteristic of the optical amplifier 20 will substantially compensate for the loss wavelength characteristic of the optical transmission line 22. In actuality, the optical amplifier of FIG. 23 is provided for every several wavelengths and the output light signals of each of these optical amplifiers are combined to compensate for the loss wavelength characteristic of the optical transmission line 22.
Slope Compensation Control and Flattening Control by Raman Amplifiers
A Raman amplifier produces gain in a signal wavelength that has been shifted from the wavelength of the excitation light by the amount of the Raman shift in the amplifying medium, as shown in FIG. 24. The amount of Raman shift and the Raman band are specific to the amplifying medium. Accordingly, if the excitation wavelength is shifted to the long-wavelength side, then the center wavelength of the gain and the gain band will be shifted toward the long-wavelength side by an amount identical with the amount of shift of the excitation wavelength. Further, optical amplification over a wide band is possible, as shown in FIG. 25, by inputting excitation light sources, which have slightly different excitation wavelengths from one another, to the amplifying medium collectively. Further, since gain varies in such a manner that the higher the power of wavelength of the excitation light, the greater the gain, any gain characteristic can be assigned to a Raman amplifier by controlling the power of each excitation wavelength (see the specification of Japanese Patent Application Laid-Open No. 2002-72262).
Tilt compensation control by a Raman amplifier is of two types. First tilt compensation control is slope compensation control. This is a function (feed-forward control) for calculating the amount of slope (amount of tilt) of the wavelength characteristic from the wavelength multiplexing number of the input optical signal and the distance along the optical transmission line in the interval that is to undergo compensation, finding the excitation ratio from the amount of slope and amplifying the input spectrum of the optical amplifier, which is connected to the Raman amplifier, while flattening the spectrum. FIG. 26 is a block diagram illustrating slope compensation control by a Raman amplifier. Here a plurality of optical signals are wavelength-division multiplexed and input to a back-excited Raman amplifying medium 31 from the input side of a Raman amplifier 30. A wavelength-division multiplexer 32 multiplexes excitation light of wavelengths λp1 to λp3 from excitation light-source blocks 33a, 33b, 33c, respectively, having different center wavelengths, and inputs the multiplexed signal to a combining coupler 34. The latter combines the excitation light of wavelengths λp1 to λp3 and the wavelength-multiplexed signal obtained by wavelength multiplexing light of a plurality of main signals, and supplies the combined signal to the Raman amplifying medium 31. An excitation light controller 35 calculates the amount of slope (the amount of tilt of each wavelength) of the wavelength characteristic from the multiplexing number of the wavelength-division multiplexed signal, which have been obtained by wavelength-division multiplexing the light of the main signals, and the distance along the optical transmission line of the interval to undergo compensation, calculates the power of each excitation light signal so as to obtain a characteristic that will be the inverse of the wavelength characteristic and inputs the power to the excitation light-source blocks 33a, 33b, 33c. As a result, the excitation light-source blocks 33a, 33b, 33c generate excitation light of the wavelengths λp1 to λp3 having an intensity conforming to the input power, and correct the tilt that is generated in the optical transmission line in the interval that undergoes compensation.
FIG. 27 is a block diagram illustrating flattening control of a Raman amplifier. Components identical with those shown in FIG. 26 are designated by like reference characters. This arrangement (feedback control) differs in that with flattening control, the slope (tilt) of the wavelength characteristic at the input of the optical amplifier (not shown) (the output of the Raman amplifier) is detected by a spectrum analyzer 37 and the tilt is corrected to achieve flattening. In FIG. 27, the excitation light controller 35 calculates the slope (tilt) of the wavelength characteristic from the output of the spectrum analyzer 37, calculates the power of each excitation light signal so as to obtain a characteristic that will be the inverse of the wavelength characteristic and inputs the power to the excitation light-source blocks 33a, 33b, 33c. As a result, the excitation light-source blocks 33a, 33b, 33c generate excitation light of the wavelengths λp1 to λp3 having an intensity conforming to the input power, correct the tilt that is generated in the optical transmission line in the interval that is to undergo compensation and flatten the wavelength characteristic.
In FIG. 27, the slope (tilt) of the wavelength characteristic of the wavelength-division multiplexed signal output from the Raman amplifier is detected and the tilt is flattened. In actuality, however, an optical amplifier is connected to the Raman amplifier and therefore the input spectrum to the optical amplifier is flattened while the spectrum (wavelength characteristic) at the input or output of the optical amplifier is monitored using a spectrum analyzer module.
Interchannel optical power variations and optical SNR deviations at the receiving end are minimized and high-capacity, long-haul transmission is made possible by the compensating scheme described above.
In order to satisfy the need in the marketplace for a reduction in cost per unit wavelength, there is growing demand for an ultra-long-haul transmission system wherein the spans in which a costly regenerator is inserted are made as long as possible and signals are transmitted in the form of light over very long distances of 2000 km and 3000 km. However, the longer the distance over which light is transmitted as is, the greater the number of transmission lines traversed, the greater the number of optical devices and the greater the number of optical amplifiers. As a consequence, channel-to-channel variations in received optical power increase.
Owing to a limitation upon amount of attenuation by the optical attenuator and a limitation upon the input level of the amplifier of the transmitted light at the transmitting end, there is a limit to the amount of tilt that can be handled by pre-emphasis control. In a case where compensation is inadequate, the optical SNR of a specific channel is degraded. To realize ultra-long-haul transmission, it is necessary to accomplish elimination of slope and beat components by the optical amplifier and Raman amplifier as much as possible at a stage prior to application of pre-emphasis in order to equalize the optical SNRs of all channels by optical pre-emphasis control under conditions where the amount of attenuation of the optical attenuator at the transmitting end is limited.
However, increasing the amount of tilt compensation by an optical amplifier or Raman amplifier, etc., and improving the optical characteristic in order to achieve ultra-long-haul transmission are mutually contradictory. For example, if amount of slope compensation is increased in the case of an optical amplifier, allowance must be made for superfluous excitation power. As a result, the noise figure (NF) is degraded and this can lead to a decline in overall optical SNR at the receiving end. Further, if flattening of the input to the optical amplifier of the next stage is given priority in the case of a Raman amplifier, there is a possibility that the input level to the optical amplifier will decline. This also leads to worsening of the optical SNR.
In order to realize even a slight improvement in the optical SNR characteristic in an ultra-long-haul transmission system, the general practice is to give priority to the noise figure or to the gain of the Raman amplifier (the input level to the optical amplifier). Accordingly, to compensate for tilt, a method of performing tilt compensation at nodes along the way has been adopted, as by using a compensating device such as an active GEQ (active gain equalizer). However, the cost is high and there is the danger that the optical SNR characteristic will be degraded by introducing such compensating devices. For these reasons, it is necessary to devise some way of minimizing the insertion of tilt compensators, or in other words, to extend, as much as possible, the distance up to the point at which a tilt compensator is inserted.
In order to lighten the load on optical pre-emphasis control, give priority to improvement of the optical SNR and carry out tilt compensation by optical amplifiers and Raman amplifiers effectively, tilt compensation by the optical amplifiers and Raman amplifiers should not be performed independently. Control and management that will apply the required amount of tilt compensation to the required location in terms of the total system are necessary.